Since you have a single mast you would need a load calculation (not adding 200+100) for both dwellings. Even though your upgrading the service technically if you're not adding any load the existing conductors can likely remain. If load were added in the future then the conductor size would need to be addressed at that point.
But you still need to provide wire that is capable of handling the potential maximum apply by to each. Thus wouldn't you need to upgrade the wire to the load potential? (this case 200+100).
The change by the OP would likely result in 2- 200A meter installation. (Not aware of a 200/100A meter stack.) And if already doing it, the Labor side wouldn't change much to provide the capacity for 2-200A, and materials only slightly different between 300A and 400A. Also If upgrade of equipment is being sought to set up for need to future corrections with a relatively small change has no advantage and can lead to HO hard feelings when it is later needed. Seen this sort of issue many times where someone saw 2-200A and felt they could up the 100 to a 200 not even looking at the service wire. (before you even say it I know we don't and can't always, and not required to "future proof" an installation)
The math involved would only realistically be helpful if you are trying to fit in a lessor size wire and then limiting the max current on the wiring, one means to do this is to have a lower back feed breaker on each. No real advantage on the small resi systems. Most homes here until you start adding a lot of EVSE and full electrification are only using about 80A max, even on a full boat 200A panel. But still wire to the 83% 200A.
